This invention relates generally to detecting isolation faults, and, more particularly, to a method and apparatus for detecting isolation faults in high-voltage motor/inverter systems.
Electric vehicle high-voltage systems continually monitor the vehicle""s direct current (DC) high-voltage bus electrical isolation from vehicle chassis ground. If this isolation is lost appropriate action can be taken, e.g., the high-voltage bus is typically disconnected. Existing systems measure isolation measure isolation loss by comparing voltage levels on the positive side to voltage levels on the negative side of the high-voltage bus. Once a predetermined threshold is reached an isolation loss is believed to have been detected and appropriate action is taken. The high-voltage supplied to various motors on a vehicle (e.g., a compressor motor, a power steering motor or a drive motor) is typically switched on and off to create an alternating current (AC) input for the motor. In certain situations ground faults may be challenging to detect, depending upon factors such as AC frequency of this motor input, capacitance level across the DC bus and a loss of AC isolation.
Existing ground fault circuit interrupters have a limited bandwidth of sensitivity for preventing erroneous detection of normal electrical noise, which would otherwise result in an undesired circuit interruption. With such systems isolation faults are generally detected upon closing of an inverter switch.
The present invention offers advantages and alternatives over the prior art in providing a method for detecting an isolation fault in a system comprising a source connected to an inverter through a circuit interrupter and the inverter connected to windings of an electric motor for driving the electric motor. The method comprising operating the inverter to yield a zero net current in the windings of the electric motor, other than leakage current resulting from an isolation fault, and measuring a voltage in the system, wherein a voltage level measured during the operation of the inverter to yield the zero net current in the windings of the electric motor is indicative of the existence of an isolation fault. Alternatively, the method comprises operating the inverter to apply a voltage to at least one of the windings of the electric motor for a period of time given an ohmic level of an isolation fault to be detected, and measuring a voltage in the system, wherein a voltage level measured during the operation of the inverter to apply the voltage to at least one of the windings of the electric motor for the period of time is indicative of the existence of the isolation fault. Thereafter, the circuit interrupter is actuated in response to the voltage level indicative of the existence of the isolation fault.